In over half the cases of aplastic anemia, the etiology is unknown. However, a role for disordered immunoregulation is suggested by frequent hematologic responses to immunosuppressive therapy. A subset of suppressor lymphocytes have been shown to mediate marrow suppression in 7 patients through a soluble mediator (19). Ten of 24 patients were found to have increased circulating and marrow IFN-gamma levels (22). However, the etiologic significance of elevated IFN-gamma levels in aplastic anemia is unclear. Colony-inhibiting lymphokine (CIL) is a novel lymphokine product of unstimulated T-lymphoblastoic cell lines with potent myelosuppressive activity and apparent Mr of 45,000 dalton by SDS PAGE. Susceptibility of hemopoietic tumor and stem cells to CIL correlates with HLA-DR antigen expression. Its production by 7 of 7 T-lymphoblastoid cell lines and ability to inhibit hemopoiesis in the 10-13 M range suggest that CIL may play a critical role in the down-regulation of hemopoiesis and contribute to myelo-suppression. The aims of the proposed research are: 1) To define the biochemical characteristics of CIL, 2) To molecularly clone the cDNA of CIL, and express it using suitable vectors, 3) To characterize the CIL-target cell interaction and examine the role of HLA-DR antigen in that interaction and 4) To examine the role of CII in normal and disordered hemopoiesis. After protein purification has been confirmed, the bioactivity of CIL will be examined in vitro, the amino acid sequence determined, and rabbit antisera generated. Molecular cloning of CIL will be accomplished through screening of expression libraries in E. coli using antisera and through screening with oligonucleotide probes or testing of supernatants of transfected COS-1 cells for bioactivity. The CIL-target cell interaction will be examined using a 125I-CIL binding assay and the role of HLA-DR examined directly using anti-HLA-DR antibodies. The possibility of a more indirect role of HLA-DR in CIL-target interaction will be examined using HLA-DR negative variants of susceptible targets. The effect of CIL on the in vivo regulation of hemopoiesis will be investigated in the mouse. Finally, blood and bone marrow samples from patients at risk for infection because of aplastic anemia and unexplained leukopenia will be examined for CIL levels. The ability of their T-lymphocytes and T-lymphocyte subsets to produce CIL and CIL mRNA will be determined.